Over 28 million musculoskeletal injuries are treated annually in the US, including approximately 2 million fracture fixation surgeries. While fixation is highly effective, refracture, malunion and non-union can occur, especially if load bearing begins before proper healing. Other potential problems include loosening from infection (5-10% of cases) and asceptic loosening. All of these problems relate to the mechanics of the implant and bone, and methods to detect the in situ stress are important in determining proper treatment course.
There are two known classes of fixation devices: external fixation devices, where pins pass through the skin and are locked with an external plate, and internal devices, with all materials implanted. External devices can be easily instrumented with electronic strain sensors and bone stiffness can be measured by applying known stresses to the pins and measuring the resulting strain. These external measurements are a highly promising diagnostic for bone healing. The rate of stiffening has been used to diagnose non-union, and using specific stiffness endpoints can reduce the average time until load bearing by weeks while also reducing the number of refractures due to premature load bearing in unhealed fractures. However, external fixation devices are more prone to infection than internal fixation devices and are less often used. For internal fixation devices, currently available strain sensors (e.g. resistive and capacitive strain gauges, fiber optic Bragg gratings, ultrasound of liquid-filled cavities, X-ray diffraction, optical moiré pattern analysis, and video tracking) are either unsuitable for non-invasive transdermal measurements or require relatively large and complex devices for power, detection, and telemetry.
In addition to fracture fixation, mechanical measurements are also important in many other biomedical applications, including tendon repair. For example, the ability to make direct assessments of the mechanical capabilities of a tendon may help to prevent failures in various types of tendon repairs, such as rotator cuff repairs.
There are various known methodologies for measuring strain and displacement in the absence of tissue, such as various optical-based methods. Specifically, Moiré pattern analysis and photoelastic polarimetry are used to map strain fields, while video tracking is used to track the position of individual mechanical components. For example video tracking was used to measure the position of dots drawn on a tendon during stress in a cadaveric model in order to determine strain on the implant. The position and velocity of motors and stages are also often measured using the reflection from optically patterned rotational and linear optical encoders. In addition, strain indicating bolts are known that include a component that changes color based upon displacement of fluid during bolt elongation and relative displacement of two components. However, it has been found that the above optical techniques are insufficient for measuring implanted medical devices through tissue for three principle reasons. First, when using any of such optical techniques, most of the incident ambient light reflects directly from the skin and superficial tissue providing a large background that obscures the very dim signal from light which penetrates through the tissue, reflects from the optical strain gauge, and penetrates back through the tissue and skin. Second, even if there is no background, optical scattering of the reflected light in the tissue results in blurring of the image, with a point spread function approximately equal to the depth through the tissue. Third, the point spread function depends significantly upon the sample orientation and depth. Thus strain measurements that depend on optical imaging will not work through tissue.
Electrical impedance and optical fiber strain gauges have also been developed for studying dynamic strain in vivo, but these require transdermal wires which can easily lead to infection. In addition, wireless devices have been developed, but these require complex electronics for power, sensing, and telemetry, which limits the size and necessitates significant modification of the implants. Non-invasive methods usually rely upon tracking the position of fiduciary markers using X-ray or ultrasound imaging, but these are ineffective at measuring displacements less than about 100 micrometers (μm). X-ray images also require acquisition at multiple angles to account for changes in sample position and angle.
What are needed in the art are strain gauges that allow for displacement and/or strain on musculoskeletal structures and/or implantable devices to be measured optically through living tissue without the need for invasive technologies so as to limit patient stress and infection opportunities.